In recent years, there has been a dramatic increase in the use of anisotropic media in structural applications such as in the aerospace industry. This anisotropy may be due to the use of reinforcing fibers in composites, directionally solidified materials or through the use of large scale single crystals in a composite matrix. Many tools that have been hitherto used for material property characterization are not sufficiently sensitive to locate defects present in directionally dependent materials. For example, conventional ultrasonic techniques have been used to locate defects in isotropic media but require a mechanical raster scan and are expensive, time consuming, and somewhat difficult to implement, particularly in manufactured articles having a complex curvature such as airfoils. To improve sensitivity in locating defects in attenuative media and articles with complex curvatures, focusing techniques are commonly used. Physical transducers are generally employed to facilitate focusing. These physical transducers employ a lens designed to coherently add acoustic signals from a localized target region in the manufactured article and simple shapes where simple lenses suffice. This approach is inadequate for use in anisotropic media.
Laser ultrasound involves the generation or detection of defects in manufactured articles by using lasers. The technique offers the potential of rapid, non-contact inspection. Typically, a laser source produces sound at a localized spot on the surface while a probe laser beam detects surface displacements or velocity. The detection is accomplished essentially at a point, resulting in unfocused detection. This method of detection is therefore not suitable for detecting defects in articles having complicated structures without suitable signal processing.
It is therefore desirable to determine methods wherein defects present in anisotropic structures having complicated articles and surfaces can be easily evaluated. It is further desirable to determine methods that can be advantageously used to characterize potential defects in anisotropic media during a real time examination in a manufacturing process.